Leptoproduction of heavy quarks. II. A unified QCD formulation of charged and neutral current processes from fixed-target to collider energies.

A unified QCD formulation of leptoproduction of massive quarks in charged current and neutral current processes is described. This involves adopting consistent factorization and renormalization schemes which encompass both vector-boson-gluon-fusion ( flavor creation'') and vector-boson-massive-quark-scattering ( flavor excitation'') production mechanisms. It provides a framework which is valid from the threshold for producing the massive quark (where gluon fusion is dominant) to the very high energy regime when the typical energy scale [mu] is much larger than the quark mass [ital m][sub [ital Q]] (where the quark scattering should be prevalent). This approach effectively resums all large logarithms of the type [[alpha][sub [ital s]]([mu])ln([mu][sup 2]/[ital m][sub [ital Q]][sup 2])][sup [ital n]] which limit the validity of existing fixed-order calculations to the region [mu][similar to][ital O]([ital m][sub [ital Q]]). We show that the (massive) quark-scattering contribution (after subtraction of overlaps) is important in most parts of the ([ital x],[ital Q]) plane except near the threshold region. We demonstrate that the factorization scale dependence of the structure functions calculated in this approach is substantially less than those obtained in the fixed-order calculations, as one would expect from a more consistent formulation.